This invention relates to fuel-cell-powered electric vehicles, and more particularly to the thermal management of such vehicles.
The automobile industry is investigating the commercial feasibility of powering electric vehicles with hydrogen-oxygen fueled fuel cells, and particularly with so-called PEM fuel cells (a.k.a. SPE fuel cells). PEM fuel cells are well known in the art, and comprise a membrane-electrode-assembly which is a thin, proton-transitive, solid polymer membrane-electrolyte (e.g. perflourinated sulfonic acid) having an hydrogen electrode (i.e. anode) on one of its faces and an oxygen electrode (i.e. cathode) on the opposite face. The hydrogen is preferably provided by catalytically decomposing liquid hydrocarbons (e.g. methanol, gasoline etc.)into H2and CO2in a reactor known as a xe2x80x9cfuel processorxe2x80x9d. Fuel processors can take several different forms, but generally comprise a steam reformer section where the hydrocarbon and steam react endothermically to form a reformate that includes H2, CO2, and some CO. The heat for the reaction is provided from either (1) an external combuster that burns a fuel to produce a heated exhaust stream that heats the reformer, or (2) a partial oxidation (POx) reactor, upstream of the steam reformer, that preheats the hydrocarbon-steam inputs to the steam reformer. The fuel processor also includes a CO clean-up section that reduces the CO content of the reformate to a sufficiently low level that it will not poison the anode catalyst of the fuel cell. The fuel processor""s clean-up section typically includes: (1) a water-gas-shift reactor that exothermically reacts the CO in the reformate with water to form more H2; and (2) a preferential oxidation (PrOx) reactor that selectively exothermically reacts the CO in the reformate with oxygen from the air. The CO clean up typically reduces the CO content to below about 50 PPM which the fuel cell can tolerate. This substantially CO-free is then sent to the fuel cell where it electrochemically and exothermically reacts with oxygen (from air) to produce electricity for powering the vehicle""s traction motor(s). The traction motor(s) and the power electronics in the controller that controls the motor(s) are both exothermic devices in that they produce heat while in use, and must be cooled.
Heat management in fuel-cell-powered vehicles is a challenge. A number of the vehicle""s components are exothermic devices in that they produce heat while in use and require cooling. Other of the vehicle""s components are endothermic devices in that they require heat to be operational. For example, the fuel cell system will typically include a number of endothermic and exothermic devices such as an air compressor (exothermic), water recovery condensers (exothermic), and vaporizers (endothermic) for vaporizing water and/or fuel for use in the system, as well as a variety of other devices that either require (endothermic) or generate (exothermic) heat. Still further, the vehicle requires a heating, ventilation and cooling subsystem (HVAC)for occupant comfort. Moreover, significant differences exist between the operating temperatures of the vehicle""s components. In this regard for example, the fuel cell, traction motor, and power electronics are typically maintained at relatively low operating temperatures in the range of about 80xc2x0 C. to about 100xc2x0 C., while the fuel processor and fuel/H2O vaporizers are maintained at relatively high operating temperatures in the range of about 200xc2x0 C. to about 300xc2x0 C.
Heretofore it has been the practice to provide several discrete heat transfer circuits one for the fuel cell system, one for the traction motors and power electronics and one for the HVAC system. Each system had its own componentry (e.g. plumbing, pumps, and valves), was completely isolated from the other systems, and used a heat transfer medium adapted to itself and different from the heat transfer mediums used in the other systems. Such componentry adds weight and cost to the vehicle.
The present invention is directed to an efficient, low weight and cost effective thermal management system for a fuel-cell-powered vehicle, which system utilizes the same heat transfer medium throughout, and minimizes the number of components required to manage the heat produced by the vehicle.
The present invention involves a fuel-cell-powered vehicle that has a fuel cell system for generating electricity from hydrogen and oxygen, a traction motor energized by the electricity to propel the vehicle, power electronics that control the traction motor, a heat exchanger that controls the environment in the vehicle""s occupant compartment , and a radiator that expels excess heat generated by the vehicle to the ambient. The invention contemplates such a vehicle having: (1) a high temperature heat transfer circuit that includes a heat-generating fuel processor that converts a liquid hydrocarbon into hydrogen for fueling a PEM fuel cell, at least one endothermic device that extracts heat from the high temperature circuit, and a first pump that circulates a dielectric liquid heat transfer medium through the high temperature circuit; (2) a low temperature heat transfer circuit that includes the fuel cell, traction motor, power electronics, radiator, and a second pump that circulates the same dielectric heat transfer medium as is used in the high temperature circuit through the low temperature circuit; (3) a controllable first valve that communicates the high and low temperature circuits and is adapted, when open, to direct a first quantity of medium from one of the circuits (i.e. the donor circuit) into the other of the circuits (i.e. the receiving circuit); (4) a second valve that communicates the high and low temperature circuits and is adapted to direct a second quantity of the medium, equal to the first quantity, from the other (i.e. receiving) circuit to the one (i.e. donor) circuit when the first valve is open; and (5) a controller, responsive to the thermal requirements of the vehicle, for controlling the opening and closing of the first valve to change the temperature of the medium in each of the circuits as dictated by the thermal needs of the components in those circuits. Preferably, the vehicle utilizes a single motor to drive both the first and second pumps.
In accordance with one embodiment of the invention, the endothermic device comprises a vaporizer for vaporizing the hydrocarbon and/or water utilized in the fuel cell system. In another embodiment of the invention, the fuel cell includes a sensor for determining its temperature and the controller is responsive to that sensor to direct hot heat transfer medium from the high temperature circuit into the low temperature circuit when the fuel cell is undesirably cold (e.g. to thaw out the fuel cell after it has sat idle at subfreezing temperatures). According to still another embodiment of the invention, the fuel processor has a sensor for determining the temperature of the fuel processor, and the controller is responsive to that sensor to direct hot heat transfer medium from the high temperature circuit into the low temperature circuit when any part of the fuel processor (e.g. the combuster or POx sections) is too hot (e.g. when the electrical load is removed from the fuel cell before the fuel processor can slow down H2 production) to extract excess heat from the medium by means of the radiator in the low temperature circuit. A particularly effective heat transfer medium comprises a dielectric oil which is liquid at 300xc2x0 C., pumpable at xe2x88x9240xc2x0 C., and has a DC volume resistivity of at least about 250 ohm-cm (i.e. as determined by ASTM Specification D-1169) in order to prevent any short circuiting of the fuel cell, or current leakage therefrom to the rest of the vehicle, via the heat transfer medium. A preferred such heat transfer medium is a paraffinic hydrocarbon having a DC volume resistivity of 1xc3x971012 ohm-cm that is sold by the Paratherm Corporation under the tradename PARATHERM OR(trademark).
In accordance with another aspect of the invention, the invention contemplates a method of operating a fuel-cell-powered vehicle having (a) a fuel cell for generating electricity from hydrogen and oxygen, (b) a traction motor energized by the electricity for propelling the vehicle, (c) power electronics for controlling the traction motor, (d) a heat exchanger for controlling the environment in an occupant compartment of the vehicle, (e) a radiator for expelling excess heat from the vehicle, (f) a heat-generating fuel processor for converting a liquid hydrocarbon into hydrogen for fueling the fuel cell, (g) at least one endothermic device that extracts heat from the vehicle, (h) a first pump for circulating a liquid heat transfer medium in a high temperature circuit between the fuel processor and the endothermic device, and (i) a second pump for circulating the heat transfer medium in a low temperature circuit through the fuel cell, the traction motor, the power electronics, and the radiator. According to one embodiment, the method invention includes the steps of: determining the temperature of the fuel cell; pumping a first quantity of the heat transfer medium from the high temperature circuit into the low temperature circuit to elevate the temperature of the medium in the low temperature circuit and warm-up the fuel cell when it is too cold (e.g. thaw out the fuel cell); and displacing an equal quantity of the medium from the low temperature circuit into the high temperature circuit when the first quantity is being pumped into the low temperature circuit . This same technique can be used to thaw out other components of the fuel cell system especially those that contain ice. According to another embodiment, the method invention is similar to that described above for thawing-out the fuel cell and H2O-containing devices, but instead is directed to providing extra cooling for an overheating fuel processor component (e.g combuster or POx) and comprises the steps of: determining the temperature of the fuel processor; and pumping a first quantity of the medium from the high temperature circuit into the low temperature circuit and an equal quantity of the medium from the low temperature circuit into the high temperature circuit to thereby cool the fuel processor.